1. Field of the Invention
The present invention relates to filter elements of the pleated kind, that is to say of the kind in which the filter element is in the form of a sheet which is pleated and curved into the form of a cylinder.
2. Description of the Prior Art
It has already been proposed to improve the performance of pleated-sheet material filter elements by arranging support elements between the pleats with a view to maintaining the pleats at a predetermined spacing and preventing the pleats moving into contact with each other and thereby reducing the effective surface area of the filter element. Examples of such previously proposed arrangements are shown in U.S. Pat. Nos. 2,914,179 and 2,988,227 and French Pat. No. 1,396,322. These known constructions are relatively expensive to manufacture since they either require the support elements to be inserted by hand into the pleated sheet after it has been formed into a cylinder or they require the individual pleats to be engaged by hand between adjacent support elements. Moreover, with these known arrangements, the greater the support given to the pleats the greater is the obstruction they offer to fluid flow.